Post on 03-Apr-2018
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Welding Technology
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JOINING
Soldering Produces coalescence of materials by heating to soldering temperature
(below solidus of base metal) in presence of filler metal with liquidus 450C
Welding Process of achieving complete coalescence of two or more materials
through melting & re-solidification of the base metals and filler metal
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Soldering & Brazing
Advantages
Low temperature heat source required
Choice of permanent or temporary joint
Dissimilar materials can be joined Less chance of damaging parts
Slow rate of heating & cooling
Parts of varying thickness can be joined Easy realignment
Strength and performance of structural joints needcareful evaluation
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Welding
Advantages
Most efficient way to join metals
Lowest-cost joining method
Affords lighter weight through better utilization
of materials
Joins all commercial metals
Provides design flexibility
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Weldability
Weldability is the ease of a material or acombination of materials to be welded underfabrication conditions into a specific, suitablydesigned structure, and to perform satisfactorily in
the intended service
Common Arc Welding Processes Shielded Metal Arc Welding (SMAW)
Gas Tungsten Arc Welding (GTAW) or, TIG
Gas Metal Arc Welding (GMAW) or MIG/MAG Flux Cored Arc Welding (FCAW)
Submerged Arc Welding (SAW)
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WELDABILITY OF STEELS
Cracking & Embrittlement in Steel Welds
Cracking
Hot Cracking
Hydrogen Assisted Cracking
Lamellar Tearing
Reheat Cracking
Embrittlement Temper Embrittlement
Strain Age Embrittlement
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Hot Cracking
Solidification Cracking
During last stages ofsolidification
Liquation Cracking
Ductility Dip Cracking
Ductility 0 Caused by segregation
of alloying elements likeS, P etc.
Mn improves resistanceto hot cracking
Formation of (Fe,Mn)S instead of FeS
Crack
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Prediction of Hot Cracking
Hot Cracking Sensitivity
HCS = (S + P + Si/25 + Ni/100) x 103
3Mn + Cr + Mo + V
HCS < 4, Not sensitive
Unit ofCrack Susceptibility
[for Submerged Arc Welding (SAW)]
UCS = 230C + 90S + 75P + 45Nb 12.3Si
4,5Mn 1
UCS 10, Low risk
UCS > 30, High risk
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HydrogenAssisted Cracking
(HAC) Cold / Delayed Cracking
Serious problem in steels
In carbon steels
HAZ is more susceptible
In alloy steels
Both HAZ and weld metal are susceptible
Requirements for HAC
Sufficient amount of hydrogen (HD)
Susceptible microstructure (hardness) Martensitic > Bainitic > Ferritic
Presence of sufficient restraint Problem needs careful evaluation
Technological solutions possible
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Methods of Prevention
of HAC By reducing hydrogen levels
Use of low hydrogen electrodes
Proper baking of electrodes
Use of welding processes without flux
Preheating
By modifying microstructure Preheating
Varying welding parameters
Thumb rule (based on experience / experimental
results): No preheat if:
CE < 0.4 & thickness < 35 mm
Not susceptible to HAC if HAZ hardness < 350 VHN
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Graville Diagram
Zone I
C < ~0.1%
Zone II
C > ~0.1%
CE < ~0.5
Zone III
C > ~0.1%
CE > ~0.5
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Determination of
Preheat Temperature (#1/2) Hardness Control Approach
Developed at The Welding Institute (TWI) UK
Considers
Combined Thickness HD Content
Carbon Equivalent (CE)
Heat Input
Valid for steels of limited range of composition In ZoneII of Graville diagram
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Hydrogen Control Approach
For steels in Zones I & III of Graville diagram
Cracking Parameter
PW = Pcm + (HD/60) + (K/40) x 104, where
Weld restraint, K = Ko x h, with h = combined thickness
Ko 69 T (C) = 1440 PW 392
Determination of
Preheat Temperature (#2/2)
BVNiCrCuMnSiCPcm
515602030
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HAC in Weld Metal
If HD levels are high
In Microalloyed Steels
Where carbon content in base metal is low
Due to lower base metal strength
In High Alloy Steels (like Cr-Mo steels)
Where matching consumables are used
Cracking can take place even at hardness as low
as 200 VHN
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Lamellar Tearing
Occurs in rolled or forged(thick) products
When fusion line isparallel to the surface
Caused by elongatedsulphide inclusions(FeS) in the rollingdirection
Susceptibility determinedby Short Transverse Test If Reduction in Area
>15%, Notsusceptible
< 5%, Highly
susceptible
Crack
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Reheat Cracking
Occurs during PWHT
Coarse-Grain HAZ most susceptible
Alloying elements Cr, Mo, V & Nb promote
cracking
In creep resistant steels due to primary creep
during PWHT !
Variation: Under-clad cracking in pipes and plates clad
with stainless steels
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Reheat Cracks
Crack
Crack
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Reheat Cracking(contd.)
Prediction of Reheat Cracking
G = Cr + 3.3 Mo + 8.1V + 10C 2
Psr= Cr + Cu + 2Mo + 10V + 7Nb + 5Ti 2
IfG, Psr> 0, Material susceptible tocracking
Methods of Prevention
Choice of materials with low impurity content
Reduce / eliminate CGHAZ by proper weldingtechnique
Buttering
Temper-bead technique
Two stage PWHT
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Temper-bead Techniques
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Temper Embrittlement
Caused by segregation of impurity elements at thegrain boundaries
Temperature range: 350600 C
Low toughness
Prediction J = (Si + Mn) (P + Sn) x 104
If J 180, Not susceptible For weld metal
PE = C + Mn + Mo + Cr/3 + Si/4 + 3.5(10P +5Sb + 4Sn + As) PE 3 To avoid embrittlement
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HAZ Hardness Vs. Heat Input
Heat Input is
inverselyproportional
to CoolingRate
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Cr-Mo Steels
Cr: 112 wt.-%Mo: 0.51.0 wt.-%
High oxidation & creepresistance
Further improved byaddition of V, Nb, Netc.
Application temp. range:
400550 C
Structure Varies from Bainite
to Martensite withincrease in alloycontent
Welding
Susceptible to
Cold cracking &
Reheat cracking
Cr < 3 wt.-%
PWHT required:
650760 C
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Nickel Steels Ni: 0.712 wt.-%
C: Progressively reducedwith increase in Ni
For cryogenicapplications High toughness
Low DBTT Structure
Mixture of fine ferrite, carbides& retained austenite
Welding
For steels with 1% Ni HAZ softening &
toughness reduction inmultipass welds
Consumables: 12.5%Ni
Welding (contd.)
For steels with 13.5% Ni
Bainite/martensite structure
Low HD consumables
Matching / austeniticSS
No PWHT
Temper-bead technique
Low heat input
For steels with > 3.5% Ni
Martensite+austenite HAZ Low heat input
PWHT at 650 C
Austenitic SS / Ni-baseconsumable
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HSLA Steels Yield strength > 300 MPa
High strength by Grain refinement
through
Microalloyingwith
Nb, Ti, Al,V, B
Thermo-mechanicalprocessing
Low impurity content
Low carbon content
Sometimes Cu addedto provideprecipitationstrengthening
Welding problems
Dilution from basemetal
Nb, Ti, V etc.
Grain growth inCGHAZ
Softening in HAZ
Susceptible to HAC
CE and methods to
predict preheattemperature are oflimited validity
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STAINLESS STEELS
SS defined as Iron-base alloy containing > 10.5% Cr & < 1.5%C
Based on microstructure & properties
5 major families of SS
Austenitic SS Ferritic SS
Martensitic SS
Precipitation-hardening SS
Duplex ferritic-austenitic SS
Each family requires
Different weldability considerations
Due to varied phase transformation behaviouron cooling from solidification
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Stainless Steels(contd. 1)
All SS types
Weldable by virtually all welding processes
Process selection often dictated by availableequipment
Simplest & most universal welding process Manual SMAW with coated electrodes
Applied to material > 1.2 mm
Other very commonly used arc weldingprocesses for SS
GTAW, GMAW, SAW & FCAW Optimal filler metal (FM)
Does not often closely match base metal composition
Most successful procedures for one family
Often markedly different for another family
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Stainless Steels(contd. 2)
SS base metal & welding FM chosen based on Adequate corrosion resistance for intended use
Welding FM must match/over-match BM contentw.r.t
Alloying elements, e.g. Cr, Ni & Mo Avoidance of cracking
Unifying theme in FM selection & proceduredevelopment
Hot cracking
At temperatures < bulk solidus temperature ofalloy(s)
Cold cracking
At rather low temperatures, typically < 150 C
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Stainless Steels(contd. 3)
Hot cracking
As large Weld Metal (WM) cracks Usually along weld centreline
As small, short cracks (microfissures) in
WM/HAZ At fusion line & usually perpendicular to it
Main concern in Austenitic WMs Common remedy
Use mostly austenitic FM with small amount of ferrite
Not suitable when requirement is for
Low magnetic permeability
High toughness at cryogenic temperatures
Resistance to media that selectively attack ferrite(e.g. urea)
PWHT that can embrittle ferrite
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Stainless Steels(contd. 4)
Cold cracking Due to interaction of
High welding stresses
High-strength metal
Diffusible hydrogen Commonly occurs in Martensitic WMs/HAZs Can occur in Ferritic SS weldments embrittled by
Grain coarsening and/or second-phase particles
Remedy
Use of mostly austenitic FM (with appropriatecorrosion resistance)
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Martensitic Stainless Steels
Full hardness on air-cooling from ~ 1000 C
Softened by tempering at 500750 C
Maximum tempering temperature reduced
If Ni content is significant
On high-temperature tempering at 650
750 C Hardness generally drops to < ~ RC 30
Useful for softening martensitic SS before welding for
Sufficient bulk material ductility
Accommodating shrinkage stresses due to welding
Coarse Cr-carbides produced
Damages corrosion resistance of metal
To restore corrosion resistance after welding
necessary to
Austenitise + air cool to RT + temper at < 450 C
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Martensitic Stainless SteelsFor use in As-Welded Condition
Not used in as-welded condition
Due to very brittle weld area
Except for
Very small weldments
Very low carbon BMs
Repair situations
Best to avoid
Autogenous welds
Welds with matching FM Except
Small parts welded by GTAW as
Residual stresses are very low
Almost no diffusible hydrogen generated
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Martensitic Stainless Steels
For use after PWHT
Usually welded with martensitic SS FMs
Due to under-matching of WM strength /hardness when welded with austenitic FMs
Followed by PWHT
To improve properties of weld area
PWHT usually oftwo forms (1) Tempering at < As (2) Heating at > Af (to austenitise) +
Cooling to ~ RT (to fully harden) +Heating to < As (to temper metal to
desired properties)
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Ferritic Stainless Steels
Generally requires rapid cooling from hot-workingtemperatures To avoid grain growth & embrittlement from
phase
Hence, most ferritic SS used in relatively thingages Especially in alloys with high Cr
Super ferritics (e.g. type 444) limited to thin plate, sheet & tubeforms
To avoid embrittlement in welding General rule is weld cold i.e., weld with
No / low preheating
Low interpass temperature
Low level of welding heat input
Just enough for fusion & to avoid cold laps/other defects
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Ferritic Stainless SteelsFor use in As-Welded Condition
Usually used in as-welded condition
Weldments in ferritic SS
Stabilised grades (e.g. types 409 & 405)
Super-ferritics
In contrast to martensitic SS
If weld cold rule is followed Embrittlement due to grain coarsening in HAZ
avoided
If WM is fully ferritic Not easy to avoid coarse grains in fusion zone
Hence to join ferritic SS, considerable amountof austenitic filler metals (usually containingconsiderable amount of ferrite) are used
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Ferritic Stainless SteelsFor use in PWHT Condition
Generally used in PWHT condition
Only unstabilised grades of ferritic SS
Especially type 430
When welded with matching / no FM
Both WM & HAZ contain fresh martensite in as-
welded condition Also C gets in solution in ferrite at elevated
temperatures
Rapid cooling after welding results in ferrite inboth WM & HAZ being supersaturated with C
Hence, joint would be quite brittle
Ductility significantly improved by
PWHT at 760 C for 1 hr. & followed by rapidcooling to avoid the 475 C embrittlement
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Austenitic Stainless SteelsFor use in As-Welded Condition
Most weldments of austenitic SS BMs Used in service in as-welded condition
Matching/near-matching FMs available for many BMs
FM selection & welding procedure depend on
Whether ferrite is possible & acceptable in WM If ferrite in WM possible & acceptable
Then broad choice for suitable FM &procedures
If WM solidifies as primary ferrite
Then broad range of acceptable weldingprocedures
If ferrite in WM not possible & acceptable
Then FM & procedure choices restricted
Due to hot-cracking considerations
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Austenitic Stainless Steels(As-Welded) (contd. 1)
If ferrite possible & acceptable
Composite FMs tailored to meet specific needs
For SMAW, FCAW, GMAW & SAW processes
E.g. type 308/308L FMs for joining 304/304L BMs
Designed within AWS specification for 0
20 FN For GMAW, GTAW, SAW processes
Design optimised for 38 FN (as per WRC-1988)
Availability limited for ferrite > 10 FN
Composition & FN adjusted via alloying in
Electrode coating of SMAW electrodes
Core of flux-cored & metal-cored wires
A t iti St i l St l
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Austenitic Stainless SteelsFor use in PWHT Condition
Austenitic SS weldments given PWHT
1) When non-low-C grades are welded &Sensitisation by Cr-carbide precipitationcannot be tolerated Annealing at 10501150 C + water quench
To dissolve carbides/intermetallic compounds (-phase)
Causes much of ferrite to transform toaustenite
2) For Autogenous welds in high-Mo SS E.g. longitudinal seams in pipe
Annealing to diffuse Mo to erase micro-segregation
To match pitting / crevice corrosion resistance ofWM & BM
No ferrite is lost as no ferrite in as-welded
condition
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Austenitic SS (after PWHT)(contd. 1)
Austenitic SS
to
carbon / low-alloy steel joints Carbon from mild steel / low-alloy steel adjacent to fusion
line migrates to higher-Cr WM producing
Layer of carbides along fusion line in WM &Carbon-depleted layer in HAZ of BM
Carbon-depleted layer is weak at elevatedtemperatures
Creep failure can occur (at elevated service temp.)
Coefficient of Thermal Expansion (CTE) mismatch betweenaustenitic SS WM & carbon / low-alloy steel BM causes
Thermal cycling & strain accumulations along interface
Leads to premature failure in creep In dissimilar joints for elevated-temperature service
E.g. Austenitic SS to Cr-Mo low-alloy steel joints
Ni-base alloy filler metals used
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Austenitic SS (after PWHT)(contd. 2)
PWHT used for Stress relief in austenitic SS weldments
YS of austenitic SS falls slowly with rising temp. Than YS of carbon / low-alloy steel
Carbide pptn. & phase formation at 600700 C Relieving residual stresses without damaging
corrosion resistance on
Full anneal at 10501150 C + rapid cooling Avoids carbide precipitation in unstabilised grades
Causes Nb/Ti carbide pptn. (stabilisation) in stabilized grades Rapid coolingReintroduces residual stresses At annealing temp.Significant surface oxidation in air
Oxide tenacious on SS
Removed by pickling + water rinse + passivation
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Precipitation-Hardening SSFor use in As-Welded Condition
Most applications for Aerospace & other high-technology industries
PH SS achieve high strength by heat treatment
Hence, not reasonable to expect WM to match
properties of BM in as-welded condition Design of weldment for use in as-welded conditionassumes WM will under-match the BM strength If acceptable
Austenitic FM (types 308 & 309) suitable for
martensitic & semi-austenitic PH SS Some ferrite in WM required to avoid hot
cracking
P i it ti H d i SS F
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Precipitation-Hardening SS For
use in PWHT Condition PWHT to obtain comparable WM & BM strength
WM must also be a PH SS
As per AWS classification
Only martensitic type 630 (17-4 PH) available as
FM As per Aerospace Material Specifications (AMS)
Some FM (bare wires only) match BMcompositions
Used for GTAW & GMAW
Make FM by shearing BM into narrow strips for GTAW Many PH SS weldments light-gage materials
Readily welded by autogenous GTAW WM matches BM & responds similarly to heat
treatment
D l F iti A t iti
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Duplex Ferritic-Austenitic
Stainless Steels Optimum phase balance
Approximately equal amounts of ferrite & austenite BM composition adjusted as equilibrium structure at ~1040C
After hot working and/or annealing Carbon undesirable for reasons of corrosion resistance All other elements (except N) diffuse slowly
Contribute to determine equilibrium phase balance N most impt. (for near-equilibrium phase balance)
Earlier duplex SS (e.g. types 329 & CD-4MCu) N not a deliberate alloying element
Under normal weld cooling conditions Weld HAZ & matching WMs reach RT with very little
Poor mechanical properties & corrosion resistance For useful properties
welds to be annealed + quenching To avoid embrittlement of ferrite by / other
phases
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Duplex SS(contd. 1)
Over-alloying of weld metal with Ni causes Transformation to begin at higher temp. (diffusion very rapid)
Better phase balance obtained in as-welded WM Nothing done for HAZ
Alloying with N (in newer duplex SS)
Usually solves the HAZ problem With normal welding heat input & ~0.15%Ni
Reasonable phase balance achieved in HAZ N diffuses to austenite
Imparts improved pitting resistance If cooling rate is too rapid
N trapped in ferrite Then Cr-nitride precipitates
Damages corrosion resistance Avoid low welding heat inputs with duplex SS
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Duplex SSFor use in As-Welded Condition
Matching composition WM Has inferior ductility & toughness
Due to high ferrite content
Problem less critical with GTAW, GMAW (but significant)
Compared to SMAW, SAW, FCAW
Safest procedure for as-welded condition Use FM that matches BM
With higher Ni content
Avoid autogenous welds
With GTAW process (esp. root pass)
Welding procedure to limit dilution of WM by BM Use wider root opening & more filler metal in the root
Compared to that for an austenitic SS joint
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Duplex SS (As-Welded)(contd. 1)
SAW process Best results with high-basicity fluxes
WM toughness
Strongly sensitive to O2 content
Basic fluxes provide lowest O2 content in WM GTAW process
Ar-H2 gas mixtures used earlier
For better wetting & bead shape
But causes significant hydrogen embrittlement
Avoid for weldments used in as-welded condition
SMAW process (covered electrodes)
To be treated as low-hydrogen electrodes for low alloysteels
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Duplex SSFor use in PWHT Condition
Annealing after welding Often used for longitudinal seams in pipe lengths, welds
in forgings & repair welds in castings
Heating to > 1040 C
Avoid slow heating
Pptn. of / other phases occurs in few minutes at800 C Pipes produced by very rapid induction heating
Brief hold near 1040 C necessary for phase balancecontrol
Followed by rapid cooling (water quench)
To avoid phase formation Annealing permits use of exactly matched / no FM
As annealing adjusts phase balance to near equilibrium
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Duplex SS (after PWHT)(contd. 1)
Furnace annealing Produce slow heating
phase expected to form during heating
Longer hold (> 1 hour) necessary at annealing temp.
To dissolve all phase
Properly run continuous furnaces Provide high heating rates
Used for light wall tubes & other thin sections If phase pptn. can be avoided during heating
Long anneals not necessary
Distortion during annealing can be due to Extremely low creep strength of duplex SS at annealing temp.
Rapid cooling to avoid phase
M j P bl ith ldi f
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Major Problem with welding of
Al, Ti & Zr alloys Problem
Due to great affinity for oxygen
Combines with oxygen in air to form a high melting pointoxide on metal surface
Remedy
Oxide must be cleaned from metal surface before start ofwelding
Special procedures must be employed
Use of large gas nozzles
Use of trailing shields to shield face of weld pool
When using GTAW, thoriated tungsten electrode to be
used Welding must be done with direct current electrode
positive with matching filler wire
Job is negative (cathode)
Cathode spots, formed on weld pool, scavengesthe oxide film
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ALUMINUM ALLOYS
Important Properties High electrical conductivity
High strength to weight ratio
Absence of a transition temperature Good corrosion resistance
Types of aluminum alloys
Non-heat treatable Heat treatable (age-hardenable)
N H t T t bl
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Non-Heat Treatable
Aluminum Alloys
Gets strength from cold working
Important alloy types Commercially pure (>98%) Al
Al with 1% Mn Al with 1, 2, 3 and 5% Mg
Al with 2% Mg and 1% Mn
Al with 4, 5% Mg and 1% Mn
Al-Mg alloys often used in welded construction
Heat treatable
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Heat-treatable
Aluminum Alloys
Cu, Mg, Zn & Li added to Al
Confer age-hardening behavior after suitable heat-treatment
On solution annealing, quenching & aging
Important alloy types
Al-Cu-Mg
Al-Mg-Si
Al-Zn-Mg
Al-Cu-Mg-Li Al-Zn-Mg alloys are the most easily welded
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Welding of Aluminum Alloys
Most widely used welding process Inert gas-shielded welding
For thin sheet
Gas tungsten-arc welding (GTAW)
For thicker sections Gas metal-arc welding (GMAW)
GMAW preferred over GTAW due to
High efficiency of heat utilization
Deeper penetration High welding speed
Narrower HAZ
Fine porosity
Less distortion
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Welding of Aluminum Alloys(contd...1)
Other welding processes used
Electron beam welding (EBW)
Advantages
Narrow & deep penetration High depth/width ratio for weld metal
Limits extent of metallurgical reactions
Reduces residual stresses & distortion
Less contamination of weld pool
Pressure welding
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TITANIUM ALLOYS
Important properties
High strength to weight ratio
High creep strength
High fracture toughness
Good ductility
Excellent corrosion resistance
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Titanium Alloys(contd...1)
Classification of Titanium alloys Based on annealed microstructure
Alpha alloys Ti-5Al-2.5Sn
Ti-0.2Pd
Near Alpha alloys Ti-8Al-1Mo-1V
Ti-6Al-4Zr-2Mo-2Sn
Alpha-Beta alloys
Ti-6Al-4V Ti-8Mn
Ti-6Al-6V-2Sn
Beta alloys Ti-13V-11Cr-3Al
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Welding of Titanium alloys
Most commonly used processes GTAW
GMAW
Plasma Arc Welding (PAW)
Other processes used Diffusion bonding
Resistance welding
Electron welding
Laser welding
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ZIRCONIUM ALLOYS
Features of Zirconium alloys
Low neutron absorption cross-section
Used as structural material for nuclear reactor
Unequal thermal expansion due to anisotropicproperties
High reactivity with O, N & C
Presence of a transition temperature
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Zirconium Alloys(contd.1)
Common Zirconium alloys Zircaloy-2
Containing
Sn = 1.21.7%
Fe = 0.070.20%
Cr = 0.050.15%
Ni = 0.030.08%
Zircaloy-4
Containing
Sn = 1.21.7%
Fe = 0.180.24% Cr = 0.070.13%
Zr-2.5%Nb
Weldability Demands For
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Weldability Demands For
Nuclear Industries Weld joint requirements
To match properties of base metal
To perform equal to (or better than) base metal
Welding introduces features that degrade mechanical &corrosion properties of weld metal
Planar defects
Hot cracks, Cold cracks, Lack of beadpenetration (LOP), Lack of side-wall fusion (LOF),etc.
Volumetric defects
Porosities, Slag inclusions Type, nature, distribution & locations of defects affect
design critical weld joint properties
Creep, LCF, creep-fatigue interaction, fracturetoughness, etc.
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Welding of Zirconium Alloys
Most widely used welding processes Electron Beam Welding (EBW)
Resistance Welding
GTAW
Laser Beam Welding (LBW) For Zircaloy-2, Zircaloy-4 & Zr-2.5%Nb alloys in PHWRs,
PWRs & BWRs
By resistance welding
Spot & Projection welding
EBW GTAW
Welding Zirconium Alloys
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Welding Zirconium Alloys
in Nuclear Industry For PHWR components
End plug welding by resistance welding
Appendage welding by resistance welding
End plate welding by resistance welding
Cobalt Absorber Assemblies by EBW &
GTAW
Guide Tubes, Liquid Poison Tubes etc bycircumferential EBW
Welding of Zirconium to Stainless steel by
Flash welding